Vegetable oils are typically obtained by pressing or extracting the oil seeds of plants. Properly processed vegetable oils are suitable for use in many edible oil and fat compositions destined for human consumption. Such edible oils and fats include salad oils, cooking oils, frying fats, baking shortenings, and margarines.
Vegetable oils primarily consist of triglycerides, but several other compounds are also present. Some of these additional compounds, such as diglycerides, tocopherols, sterols, and sterol esters, need not necessarily be removed during processing. Other compounds and impurities such as phosphatides, free fatty acids, color pigments, soaps, gums, albuminous matter, odoriferous volatiles, colorants, waxes, and metal compounds negatively affect taste, smell, appearance and storage stability of the refined oil, and hence must be removed.
Phospholipids, which occur in most natural fats and oils, can cause objectionable colors, odors and flavors in a finished oil product. Phosphorus and metal ions such as calcium, magnesium, iron, and copper are believed to be chemically associated with phospholipids, including phosphatides, and have deleterious effects on refined oil products. Moreover, calcium and copper can form precipitates, while iron and copper promote oxidative instability. Free fatty acids result from hydrolysis of the triglycerides of the edible oils. Color impurities typically present in oils include, for example, carotenoids, xanthophylls, xanthophyll esters, chlorophyll, tocopherols, as well as oxidized fatty acids and fatty acid polymers. Peroxides (reported as peroxide value, PV) are products of oxidation of the oil.
Oil impurities are typically removed in four distinct steps of degumming, refining, bleaching, and deodorizing. Of these four steps, degumming removes the largest amount of impurities, the bulk of which are hydratable phosphatides. Refining primarily removes non-hydratable phosphatides, soaps created from the neutralization of free fatty acids, and other impurities such as metals. Bleaching then improves the color and flavor of refined oil by decomposing peroxides and removing oxidation products, trace phosphatides, and trace soaps. Deodorizing is the final processing step and prepares the oil for use as an ingredient in many edible products. The deodorizing process generally comprises passing steam through refined oil at high temperature and under near vacuum conditions to vaporize and carry away objectionable volatile components.
Acid-activated clays of high activity are used to adsorb colored pigments (carotenoids, chlorophyll) and colorless pigments (phospholipids) from edible and inedible oils. This process is called “bleaching” and serves both cosmetic and chemical purposes. Thus, bleaching reduces color, whereby very clear, almost water white oils are produced that meet with consumer expectations. Bleaching also stabilizes the oil by removing colored and colorless pigments which tend to “destabilize” the oil (facilitate oxidation), resulting in oils that rancidify more easily if they are not removed.
The filtering of edible oils which are bleached is accomplished by filter assemblies having a diatomaceous earth precoat. The precoat is an inert material and does not contribute to bleaching performance. Before the edible oil is introduced into the tank of the filter assembly, a slurry of a precoat material, such as diatomaceous earth, in a liquid is directed into the tank. As the liquid passes through the porous media (filter screen), the precoat material is deposited on the porous media, forming a layer of precoat material on the face of the filter.
Once a sufficient layer of precoat material is deposited on the filter, the flow of slurry into the tank may be terminated and the edible oil containing the particles of bleaching clay is introduced into the tank. As the edible oil flows through the precoat layers of the filter, the particles of bleaching clay are trapped in the precoat layer. The filtered edible oil then flows through the porous media of the filter and exits the filter assembly.
The precoat layer eventually becomes fouled with the particles of bleaching clay which are filtered from the oil stream. Essentially, the build-up of particulate solids in and on the precoat increases the pressure drop across the filter, thereby degrading the performance of the filter. Consequently, the flow of edible oil into the tank of the filter assembly is periodically terminated and the filter assembly is backwashed to remove the precoat layer and the build-up of particulate solids trapped in the precoat layer. Typically, a cleaning liquid, referred to as a backwash liquid, is forced at a high flow rate, pressure, and/or volume in a reverse direction through the filter and through the porous. The backwash liquid flowing in the reverse direction to the porous medium forces the precoat layer and the particles of bleaching clay off of the porous medium so the backwash liquid and the solids material are removed. Once the filter assembly is backwashed, another cycle of introducing the precoat slurry into the tank to form the precoat layer, introducing the contaminated edible oil into the tank to remove the particles of bleaching clay, and backwashing the fouled precoat layer is begun.
While this process of filtering edible oils through a leaf filter assembly is very effective for removing particles of bleaching clay, it nonetheless has several problems. For example, this process generates a huge amount of contaminated waste. Though the quantity of precoat material is small compared to the quantity of bleaching clay (about 5 to 40 parts clay per part precoat), all of the solids and contained oil must be properly disposed of once it is flushed from the filter assembly. Further, once the spent precoat layer is backwashed from the filters, a new precoat layer must again be deposited on each face of the filter. This is a time consuming portion of the cycle which detracts from the overall efficiency of the process since none of the edible oil is being filtered while the precoat layer is being re-deposited. In addition, in many instances, it is difficult to remove all of the precoat from the filter.